Enable "--config-file" flag for clang-tidy

[stevenengland]

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Configuration

Operating system: Win10

PlatformIO Version (platformio --version): latest

Description of problem

As a developer I would like to tell clang-tidy to use an external config file to keep my configs clean. Especially when handling multiple clang-tidy checks the code looks messy if I am forced to use the clangtidy extra flag --checks= for that purpose.
Instead it would be great to use a config file that is made of yaml like this:

---
Checks:              '-*,some-check'
WarningsAsErrors:    ''
HeaderFilterRegex:   ''
FormatStyle:         none
InheritParentConfig: true
User:                user
CheckOptions:
  - key:             some-check.SomeOption
    value:           'some value'
...

CLang Tidy comes with a flag --config-file for that scenario (see clang-tidy --help, it is not documented on the websites). Its help text is: "Specify the path of .clang-tidy or custom config file: e.g. --config-file=/some/path/myTidyConfigFile This option internally works exactly the same way as --config option after reading specified config file. Use either --config-file or --config, not both."

Actually it is not possible to use this flag because at least the Checks part is overriden due to the logic in https://github.com/platformio/platformio-core/blob/4839fe37a3d1433737c80147ce9a205100dc4925/platformio/commands/check/tools/clangtidy.py:

if not self.is_flag_set("--checks", flags):
            cmd.append("--checks=*")

Additional info